Method of making photolithographically-patterned out-of-plane coil structures

ABSTRACT

An out-of-plane micro-structure which can be used for on-chip integration of high-Q inductors and transformers places the magnetic field direction parallel to the substrate plane without requiring high aspect ratio processing. The photolithographically patterned coil structure includes an elastic member having an intrinsic stress profile. The intrinsic stress profile biases a free portion away from the substrate forming a loop winding. An anchor portion remains fixed to the substrate. The free portion end becomes a second anchor portion which may be connected to the substrate via soldering or plating. A series of individual coil structures can be joined via their anchor portions to form inductors and transformers.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a division of copending U.S. application Ser.No. 09/573,815 filed May 17, 2000, the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention generally relates tophotolithographically-patterned, out-of-plane coil structures for use inintegrated circuits, circuit boards and other devices.

[0004] 2. Description of Related Art

[0005] Standard bonding techniques for electrically connectingintegrated circuits, or chips, to a circuit board or other deviceinclude wire bonding, tab bonding, and solder-bump flip-chip bonding.FIG. 1 shows a contact pad 3 formed on a chip 2 wire bonded to acorresponding contact pad 3 formed on a substrate 1. The contact pads 3are electrically connected, or bonded, by a wire 4. Since the chip 2typically has tens or even hundreds of the contact pads 3, wire bondingeach contact pad 3 on the chip 2 to the corresponding contact pad 3 onthe substrate 1 is labor intensive, expensive and slow. Further, thecontact pads 3 must be large enough to accommodate both the wire 4 andthe accuracy of the wire bonding device used to create the wire bond.Therefore, the contact pads 3 are made larger than otherwise necessaryto compensate for the size limitations of wire 4 and the wire bondingdevice.

[0006]FIG. 2 shows the contact pad 3 formed on the chip 2 tab bonded tothe corresponding contact pad 3 on the substrate 1. A flexible substrate5 having conductive lines formed on its lower surface is forced againstthe contact pads 3. A layer of anisotropic adhesive (not shown) isplaced between the contact pads 3 and the flexible substrate 5. When theflexible substrate 5 is pressed against the contact pads 3, theanisotropic adhesive and the conductive lines formed on the flexiblesubstrate 5 cooperate to complete the electrical connection between thecontact pads 3. Like wire bonding, tab bonding suffers from yield lossand high cost. Irregularities in the heights of the contact pad 3 resultin non-uniform contacting force pressing the flexible substrate 5against the contact pads 3. The non-uniform contacting force means thatsome contact pads 3 will not be properly bonded to the flexiblesubstrate 5.

[0007] Another conventional method for bonding the contact pads 3 formedon the chip 2 to the contact pads 3 formed on the substrate 1 or to someother device is solder-bump flip-chip bonding. FIG. 3 shows the chip 2inverted with the contact pads 3 facing toward the substrate 1. The name“flip-chip” derives from the inversion of the chip 2, since the chip 2is “flipped over” with the contacts pads 3 facing the substrate 1, incontrast to both tab bonding and wire bonding where the contact pads 3on the chip 2 face away from the substrate 1. In standard flip-chipbonding, solder bumps 6 are formed on the contact pads 3 on thesubstrate 1. The electrical connection between the corresponding contactpads 3 is completed by pressing the contact pads 3 on the chip 2 againstthe solder bumps 6.

[0008] Flip-chip bonding is an improvement over both wire bonding andtab bonding. The relatively soft solder bumps 6 tend to permanentlydeform when the chip 2 is pressed down against the solder bumps 6. Thisdeformation of the solder bumps 6 compensates for some irregularity inthe heights of the contact pads 3 and any uneven contacting pressureforcing the chip 2 against the solder bumps 6.

[0009] However, flip-chip bonding does suffer from both mechanical andthermal variations in the solder bumps 6. If the solder bumps 6 are notuniform in height or if the substrate 1 is warped, contact between thecontact pads 3 and the solder bumps 6 can be broken. Also, if thecontacting pressure forcing the chip 2 down on the solder bumps 6 isuneven, contact between some contact pads 3 and corresponding solderbumps 6 can fail.

[0010]FIG. 4 shows a standard technique for establishing a temporaryelectrical contact between two devices. A probe card 7 having aplurality of probe needles 8 contacts the contact pads 3 by physicallypressing the probe needles 8 against the contact pads 3. The physicalcontact between the probe needles 8 and the contact pads 3 creates anelectrical connection between the probe needles 8 and the lines 9 formedon the substrate 1.

[0011] The probe cards 7 are generally used to create only temporarycontacts between the probe needles 8 and the contact pads 3, so that thedevice 10 can be tested, interrogated or otherwise communicated with.The device 10 can be a matrix of display electrodes which are part of anactive-matrix liquid crystal display. Testing of the devices 10, such asliquid crystal display electrode matrices, is more thoroughly describedin an application JAO 34053 to the same inventor, co-filed andco-pending herewith and herein incorporated by reference.

[0012] The probe cards 7 have many more applications than only fortesting liquid crystal displays. Any device 10 having numerous andrelatively small contact pads 3, similar to those found on the chip 2,can be tested using the probe card 7. However, standard techniques forproducing the probe card 7 are time consuming and labor-intensive. Eachprobe card 7 must be custom-made for the particular device 10 to betested. Typically, the probe needles 8 are manually formed on the probecard 7. Because the probe cards 7 are custom-made and relativelyexpensive, the probe cards 7 are not typically made to contact all ofthe contact pads 3 on the device 10 at one time. Therefore, onlyportions of the device 10 can be communicated with, tested orinterrogated at any one time, requiring the probe card 7 be moved toallow communication, testing or interrogation of the entire device 10.

[0013] The probe cards 7 are also used to test the chips 2 while thechips 2 are still part of a single-crystal silicon wafer. One such probecard 7 is formed by photolithographic pattern plated processing, asdisclosed in Probing at Die Level, Corwith, Advanced Packaging,February, 1995, pp. 26-28. Photolithographic pattern plated processingproduces probe cards 7 which have essentially the same design as thestandard probe card 7. However, this new type of processing appears toautomate the method for producing probe needles 8, thus avoidingmanually forming the probe needles 8. Also, this article discloses aprobe card 7 which is bent at the end nearest the probe needles 8, asshown in FIG. 5. The bend in the probe card 7 allows the probe needles 8to contact the contact pad 3 at an angle. As the probe card 7 pushes theprobe needles 8 into the contact pads 3, a mechanical scrubbing actionoccurs which allows the probe needles 8 to break through the oxideformed-on the top surface of the contact pad 3.

[0014] All of the standard probe cards 7, however, are limited totesting contact pads 3 which are arranged in a linear array. Also, thestandard probe cards 7 are sensitive to variations in the height of thecontact pads 3 on the substrate 1, irregularities or warping of thesubstrate 1, and temperature variations.

[0015] The integration of small inductors on silicon substrates has beenthe subject of intense worldwide research for more than 15 years. Thiseffort is driven by the desire to integrate coils on silicon and galliumarsenide integrated circuits (ICs). The structures proposed so far,however, have been variations of devices in which, due to technologicalconstraints, the coil windings have almost always been implemented asspirals parallel to the underlying substrate.

[0016] These in-plane architectures have two major drawbacks. When madeon a substrate that is slightly conducting such as silicon, the coilmagnetic fields induce eddy currents in the underlying substrate. Thesecurrents cause resistive dissipation that contributes to the coillosses. The second problem arises when the coil is operated at highfrequencies, where skin and proximity effects force the coil current toflow along the outer surfaces of the conductor. The “skin depth” isabout 2 to 3 μm for typical conductors at frequencies of interest forwireless communication, for example, 900 MHz, 1.9 GHz and 2.4 GHz. TheAC resistance of the coil conductor becomes appreciably higher than itsDC resistance because the cross section of the conductor is not fullyused.

[0017]FIG. 31 shows the current distribution in in-plane coils operatedat high frequencies. Darker shading in the coil indicates a highercurrent density. The disk-shaped coil shown in FIG. 3 la has a currentdistribution that is concentrated at the outer edges of the windingwire. Therefore, widening the conductor simply increases the unusedportion of the conductor and does not reduce the AC resistance. Thissituation may be compared to the typical discrete component,out-of-plane coil of FIG. 31b, where the AC resistance can be reduced bysimply making the conductors wider.

[0018] Solutions have been proposed and tried in the past to address thedrawbacks associated with in-plane inductor coils. Eddy currents can bereduced, for example, by etching away the substrate underneath the coil.However, this approach is not practical as it sacrifices structuralintegrity and destroys existing electronic circuitry on the siliconsubstrate. To reduce the AC resistance of the device in FIG. 31a, theconductor can be made very thick using micromachining techniques such asLIGA (see A. Rogner et al., “The LIGA technique—what are the newopportunities,” J. Micromech. Microeng., vol.2, pp. 133-140, 1992.).However, processing high aspect ratio structures is difficult andexpensive.

[0019] Various out-of-plane techniques have been suggested. For example,Chukwunenye Stanley Nnebe, in “A Mechanically-raised MicromachinedVariable Inductor Coil” (www.ee.cornell.edu/MENG/Abstracts/tien.htm)describes an out-of plane variable inductor structure. The structure isinitially gold-metallized strips of polysilicon on the surface plane ofthe substrate, which are then raised and fastened via a hinging systemto form a triangular geometry upon contact. After the setup of the coilis completed, the slider representing the magnetic core can then beactivated through an impact system that is controlled by four comb-driveresonators (two comb-drive resonators for each direction of motion). Theinsertion of the magnetic core through the coils would influence themagnetic flux developed around the coils and, thus, would vary theinductance accordingly. The tuning range of the inductor is set by thiseffect, and reliable data may be obtained by carefully controlling thefour resonators that actuate the slider causing it to move a finitedistance through the coils. Such a technique is fairly complicated tomicromachine and requires additional components on valuable chip realestate.

[0020] Robert Marcus et al. in International PCT Application number WO99/18445 filed Oct. 2, 1998, titled Michromachined element and Method ofFabrication thereof discloses a coiled structure that is formed bydepositing two layers of material having different coefficients ofthermal expansion on a sacrificial layer, removing the sacrificiallayer, then heating the cantilevered structure until it curls partiallyupon itself. Coil closure is achieved by patterning a tethered end tothe tip of the cantilevered structure.

[0021] When the sacrificial layer is removed and the cantilever heated,the cantilever curls on itself, causing the tethered end to twist. Sucha method and structure, however, is impractical for creating arrays ofdensely packed, integrated micro-inductors and other structures onsilicon substrates.

[0022] Low-loss inductors that can be integrated on chip are mostdesirable in wireless communication devices such as cellular phones,pagers, GPS receivers, warehouse management RF identification tags,wireless computer LANs, personal digital assistants, and satellitetelecommunication. Small portable devices, in particular, require thelowest possible power consumption for extended battery life and amaximal circuit integration to reduce device size and PC boardcomplexity. The quest for low-loss inductors is driven by a fundamentaltrade-off between power consumption on one hand and the need forlow-loss circuit passives (i.e. inductors and capacitors) on the other.Lowering the transistor bias in radio circuits reduces the powerdissipation, but also significantly degrades amplifier gains, oscillatorstability and filter selectivity. Using low-loss passives is the onlyviable technique to overcome this problem. Low-loss capacitors in the0.1 to 100 pF range are routinely integrated on chip nowadays. However,state-of-the-art integrated coil architectures are still too lossy to beof use in integrated RF designs. All present RF chipsets, therefore, arelimited to using discrete inductors that form a real estate bottleneckin today's increasingly miniaturized applications.

[0023] Modern wireless designs typically run in the lower GHz bands. Thestandard frequencies for cellular phones are 900 MHz, 1.8 GHz, 1.9 GHzand 2.4 GHZ, while 900 MHz is the frequency of choice for digitalcordless phones. The 410-430 MHz, 870 MHz and 900-930 MHz bands are usedfor wireless RS-232, computer LANs and RF identification. At these 100MHz to GHz frequencies, the passives of choice are typically, forinductors, 1 to 30 nH and, for capacitors, 1 to 30 pF. The intermediatefrequencies in superheterodyne receivers are 40 to 350 MHz which callsfor passives in the order of 100 to 1000 nH and 10 to 100 pF. Althoughhigh quality on-chip capacitors ranging from 0.1 pF to 100 pF arecommonplace, integrated inductors and integrated variable capacitorsthat meet the low-loss requirements are currently not available.

[0024] Variable capacitors (varicaps) that can be integrated on chip arealso in great demand. The benchmark architecture for contemporarywireless transceivers is still the superheterodyne architecture, whichuses both inductors and varicaps. Variable capacitors are essentialcomponents of superheterodyne circuits used in many wireless devices.Superheterodyne circuits containing both inductors and capacitorscurrently cannot be integrated on chip in commercial devices, and sopresent a bottleneck to device miniaturization. The missing links inimplementing full superheterodyne wireless architectures on a chip areinductors with quality factors of at least 30 to 50, variable capacitors(varicaps) with a tuning range of 10% and quality factors of 30 to 50,and oscillators with quality factors of 10,000 or more. The processtechnology for making the capacitors should be compatible with theprocess for making the inductors.

[0025] Present wireless devices use discrete off-chip components toimplement superheterodyne circuits. The very high Q oscillator isusually a crystal oscillator. There are also numerous Voltage ControlledOscillators (VCOs), each of which uses at least one discrete inductorand one discrete varicap. Because of these discrete components VCOsoccupy a large portion of many RF circuit area. Being able to integrateentire VCOs on chip requires a new type of varicap as well as inductor.

[0026] There is a need for a micromachined coil structure which is easyto manufacture and does not use a lot of chip real estate. There is aneed for low loss coil structures and variable capacitors that can beintegrated on conductive substrates, such as silicon. There is also aneed for an integrated coil structure in which the windings have lowerresistance. There is a need for a method of manufacturing a coilstructure in which closing the turns of the coil electrically produces aviable electrical structure. There is a need for a manufacturabletechnique that results in a closed coil structure suitable for high-Qintegrated passive inductor elements. There is a need for amanufacturing technique which would enable the integration of both onchip inductors and varicaps.

SUMMARY OF THE INVENTION

[0027] Accordingly, this invention provides a spring contact whichexhibits the speed and ease of solder-bump flip-chip bonding whileeliminating the need to create uniform solder bumps or uniformcontacting pressure. This invention further provides a spring contactwhich has elastic properties enabling the spring contact to maintainphysical contact with a contact pad despite variations in contact padheights, contacting pressure or thermal variations. This invention alsoprovides an elastic spring contact having a stress gradient formed inthe spring contact, which causes the spring contact to bend away fromthe substrate and thus provide compliant contact with a contact pad.This invention further provides a probe card and a method for producingthe probe card having spring contacts in place of standard probeneedles.

[0028] The spring contacts of this invention are formed of a thin metalstrip which is in part fixed to a substrate and electrically connectedto a contact pad on the substrate. The free portion of the metal stripnot fixed to the substrate bends up and away from the substrate.

[0029] When the contact pad on a device is brought into pressing contactwith the free portion of the metal strip, the free portion deforms andprovides compliant contact with the contact pad. Since the metal stripis electrically conductive or coated with a conductive material, thecontact pad on the substrate is electrically connected to the contactpad on the device via the spring contact.

[0030] Another embodiment of the invention overcomes the drawbacks ofplanar coil structures in that the coil structures of the inventionplace the coil axis parallel, rather than perpendicular, to thesubstrate plane. A coil structure, according to the invention, includesa substrate and an elastic member having an intrinsic stress profile.The elastic member includes a first anchor portion fixed to thesubstrate, a loop winding and a second anchor portion connected to thesubstrate. The second anchor portion and the loop winding are initiallyfixed to the substrate, but are released from the substrate to becomeseparated from the substrate. An intrinsic stress profile in the elasticmember biases the second anchor portion away from the substrate formingthe loop winding and causing the second anchor portion to contact thesubstrate. The resulting coil structure is out-of-the plane of thesubstrate. The loop winding may also include a plurality of turns.

[0031] Various techniques may be used to position the second anchorportion away from the takeoff point of the elastic member, eithertangentially or axially. If the second anchor point is positionedtangentially from the takeoff point, the loop winding is generally inthe shape of a circle, i.e., the second anchor portion contacts thesubstrate in the same vertical plane as the first anchor portion.Various techniques may be used to position the second anchor portiontangentially from the takeoff point. For example, a mechanical stop canbe fixed to the substrate at the desired location. Alternatively, theradius of curvature of the elastic member may be varied, such as byadding a load layer uniformly across the width of a portion of theelastic member or by patterning one or more openings or perforationsuniformly across the width of a portion of the elastic member.

[0032] If the second anchor portion is positioned axially from thetakeoff point or first anchor portion, the loop winding is generally inthe shape of a helix. Several techniques may be used to form the loopwinding in a helix. For example, a uniform stress anisotropy may beintroduced into the elastic member, which causes a helical deformationin the released layer. Alternatively, the radius of curvature can bevaried in the elastic member to introduce a helical deformation. Thiscan be accomplished by varying the intrinsic stress profile in theelastic member as a function of position. A helical winding may also beformed by causing the resulting loop winding to have two (or more)different radii of curvature. This may be accomplished, for example, byforming one or more openings asymmetrically in the elastic member priorto release or by forming a load layer at an angle on a portion of theelastic member (upon release, the weight of the load layer causes theloop winding to be axially offset).

[0033] Various techniques can be used to connect the second anchorportion to the substrate. For example, the second anchor portion can besoldered or plated to the substrate. Each anchor portion can be attachedto a metal contact pad attached to the substrate, for providingelectrical connectivity to other elements in a circuit. Preferably theelastic member is formed of a conductive material. Alternatively, alayer of a conductive metal, such as gold or silver, may be plated on aninner surface, an outer surface, or both surfaces.

[0034] This novel structure allows, for the first time, the integrationof submillimeter-size high-Q inductors on both insulating and conductivesubstrates. When fabricated on a conductive substrate like silicon, thecoil structure produces much fewer magnetic flux lines that interceptthe substrate than present in-plane micro-coils, which then results infewer eddy currents induced in the substrate and lower coil losses.Furthermore, the coil structures may be used as inductors, which arecompatible with toroidal architectures that confine magnetic fieldsexceedingly well. This property enables multiple micro-coils to bepacked densely without coupling with each other. At high operatingfrequencies, skin and proximity effects increase the coil resistance.Unlike in-plane micro-coils, the out-of-plane coil structures can beeasily designed for low resistance operation without complicated highaspect-ratio processing. The out-of-plane coil structures are alsocompatible with numerous micro-coil embodiments such as center-tappedinductors and transformers for a wide range of applications.

[0035] A method for forming a coil structure, according to theinvention, includes depositing a layer of an elastic material on asubstrate, the elastic material having an intrinsic stress profile. Thelayer of elastic material is then photolithographically patterned intoan elastic member. A portion of the substrate under the patternedstructure is under-cut etched to release a free portion of the elasticmember from the substrate, an anchor portion of the elastic memberremaining fixed to the substrate. The intrinsic stress profile in theelastic member biases the free portion of the elastic member away fromthe substrate, forming a loop winding and causing the free end tocontact a point on the substrate. This free end can then be connected tothe substrate by, for example soldering or plating.

[0036] During the removal of sacrificial layers from the substrate, theintrinsic stress bends metal containing strips into the turns of aninductor coil. Fabrication of micro-coil structures requires controlover the coil geometry, in particular the coil radius, and, if a stressanisotropy is present, the helical pitch of the coil elements as well.If, for example, the loop has a constant radius r of curvature, and thelength of the released portion is 2πr, the free end will naturallyreturn to the take-off point of the loop. By placing a mechanical stopat a position away from the take off point, the free end can bepositioned and anchored. Magnetic structures can be created with suchloops by connecting the loops on the substrate with contact pads whichextend from the take off point of one loop to the contact point of anadjoining loop, producing a spiral. In another embodiment, the free endof the spring is offset axially and/or transversely with respect to thetakeoff point in order to provide for contact to adjacent loop pads.Mechanical and electrical contact is made permanent, for example, bysoldering, conductive adhesive, thermal compressive bonding or plating.

[0037] One aspect of the invention recognizes that it is possible tocreate helical coiled structures of controlled diameter and pitch byexploiting stress anisotropy engineered into the deposited metal. Thehelical twisting provides the useful feature that the free end of to thespring is shifted longitudinally (or axially) from the take-off point.In principle, this allows formation of a continuous inductor consistingof multiple turns without interruption of the spring metal. It alsoallows segments of more than one turn to be joined in order to producean inductor. These structures reduce the number of coil-closinginterconnects, and thereby minimize the impact of interconnects oninductor quality factor.

[0038] Another aspect of this invention relates to producing a turn of acoil endowed with the property of non-constant radius. This allows thefree end of the elastic member to contact a point other than thetake-off point of the loop, either tangentially from the takeoff pointor offset axially from the takeoff point. Once a point away from thetake-off point is contacted electrically, the un-lifted metal can beused to run a trace to any other point of an electrical circuit,including to another loop of an inductor. Several ways of varying theradius of curvature are described, including varying the intrinsicstress profile along the elastic member, depositing a load layer along aportion of the elastic member, and photolithographically patterningperforations in the elastic member.

[0039] A new type of high-Q variable capacitor includes a substrate, afirst electrically conductive layer fixed to the substrate, a dielectriclayer fixed to a portion of the electrically conductive layer, and asecond electrically conductive layer having an anchor portion and a freeportion. The anchor portion is fixed to the dielectric layer and thefree portion is initially fixed to the dielectric layer, but is releasedfrom the dielectric layer to become separated from the dielectric layer.An inherent stress profile in the second electrically conductive layerbiases the free portion away from the dielectric layer. When a biasvoltage is applied between the first electrically conductive layer andthe second electrically conductive layer, electrostatic forces in thefree portion bend the free portion towards the first electricallyconductive layer, thereby increasing the capacitance of the capacitor.

[0040] The manufacturing techniques of the invention can be used tocreate a tunable LC combination employing a coil structure and variablecapacitor to provide full superheterodyne wireless architectures on asilicon chip.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] This invention will be described in relation to the followingdrawings, in which reference numerals refer to like elements andwherein:

[0042]FIG. 1 shows a chip wire bonded to a substrate;

[0043]FIG. 2 shows the chip tab bonded to the substrate;

[0044]FIG. 3 shows the chip solder-bump flip-chip bonded to thesubstrate;

[0045]FIG. 4 shows a probe card contacting an electronic device;

[0046]FIG. 5 shows a probe card having an angled probe needle;

[0047]FIG. 6 is a spring contact in an undeformed free state and anotherspring contact deformed when contacting a contact pad;

[0048]FIG. 7 shows a metal strip with no stress gradient;

[0049]FIG. 8 shows a model for determining the curvature of a springcontact due to the stress gradient;

[0050]FIG. 9 shows a model for determining the amount of reaction forceexerted at the tip of the spring contact;

[0051] FIGS. 10-13 show the steps for one method of forming a springcontact;

[0052]FIG. 14 is a graphic representation of the film stress in asputter deposited nickel-zirconium alloy as a function of plasma gaspressure;

[0053]FIG. 15 is a top view of a spring contact;

[0054]FIG. 16 is a device for testing the contact resistance of aplurality of spring contact pairs;

[0055]FIG. 17 is a graphical representation of the detected resistanceof a plurality of spring contact pairs;

[0056]FIG. 18 is a graphic representation of the contact resistance of aspring contact as a function of the distance between the contact pad andthe substrate;

[0057]FIG. 19 is a spring contact having a flat end;

[0058]FIG. 20 is a spring contact having a pointed end;

[0059]FIG. 21 is a spring contact having two points t the tip end;

[0060]FIG. 22 is spring contact having multiple points at the tip end;

[0061]FIG. 23 is a spring contact having a deformable tab at the tipend;

[0062]FIG. 24 shows a spring contact having a deformed tab end whenforced against a contact pad;

[0063]FIG. 25 is a chip having a plurality of spring contactselectrically bonded to a substrate;

[0064]FIG. 26 is a chip bonded to a dust cover and having a plurality ofspring contacts electrically contacted to a substrate;

[0065]FIG. 27 is a chip bonded to a substrate and electrically contactedto a substrate by a plurality of spring contacts on the chip having adust cover;

[0066]FIG. 28 is a chip electrically bonded to a substrate by way of anintermediate wafer having a plurality of spring contacts;

[0067]FIG. 29 is a probe card having a plurality of spring contacts usedfor testing an electronic device;

[0068]FIG. 30 is a liquid crystal display and a device for testing theoperation of the display;

[0069]FIGS. 31a and 31 b are cross-sections illustrating the currentdistribution at high frequencies in a disk shaped coil and a solenoid,respectively;

[0070]FIG. 32 is a cross-section of a stack of stress graded filmdeposited above a release layer;

[0071]FIG. 33 illustrates a constant radius coil structure;

[0072]FIG. 34 is an SEM micrograph of a series of constant radius loops;

[0073]FIG. 35 illustrates a multi-turn coil formed from a series ofconnected loops;

[0074]FIG. 36 illustrates positioning the second anchor portion with amechanical stop;

[0075]FIG. 37 illustrates positioning of the mechanical stop of FIG. 36;

[0076]FIG. 38 is a graph of tip trajectories for different elasticmember lengths for a coil radius of 100 μm;

[0077]FIG. 39 illustrates forming a multi-turn coil from individualtilted coils;

[0078]FIG. 40 illustrates a method of providing inter-coil connections;

[0079]FIG. 41 illustrates a toroidal solenoid;

[0080]FIG. 42 illustrates coil tapping;

[0081]FIG. 43 illustrates a simple air core transformer;

[0082]FIG. 44 illustrates an air-core transformer with intertwinedprimary and secondary windings;

[0083]FIG. 45 illustrates an inductor with electroplated permalloycores;

[0084]FIG. 46 illustrates laminating metallic cores;

[0085]FIGS. 47a and 47 b illustrate two stages of a micro-transformer;

[0086]FIG. 48 illustrates different helical pitch from varied springorientation;

[0087]FIG. 49 illustrates a multi-turn coil formed of single helicalturn coils;

[0088]FIG. 50 illustrates a helically joined multi-turn loop;

[0089]FIG. 51 is a plot of a three segmented spring with three differentradii;

[0090]FIG. 52 illustrates a coil closed using a load member;

[0091]FIG. 53 illustrates transversely joined single turn loops;

[0092]FIGS. 54a and 54 b illustrate two structures having a varyingradius of curvature;

[0093]FIG. 55 is a cross-section of a varicap according to theinvention;

[0094]FIG. 56 is a graph of varicap capacitance versus spring lift; and

[0095]FIG. 57 is a top view of a varicap having a large array ofindividual capacitor elements; and

[0096]FIG. 58 is a cross-section along line A-A of FIG. 57.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0097]FIG. 6 shows a side view of a bonding structure 100 having aplurality of spring contacts 15. Each spring contact 15 comprises a freeportion 11 and an anchor portion 12 fixed to an underlayer or releaselayer 13 and electrically connected to a contact pad 3. Each springcontact 15 is made of an extremely elastic material, such as achrome-molybdenum alloy or a nickel-zirconium alloy. Preferably, thespring contacts 15 are formed of an elastic conductive material,although they can be formed of a non-conductive or semi-conductivematerial if they are coated or plated with a conductor material. Morepreferably, the spring contacts 15 are formed of a nickel-zirconiumalloy having 1% zirconium. Zirconium is added to the alloy to improvethe elastic properties of the alloy while not greatly affecting theconductivity of the alloy. When the elastic material is not conductive,it is coated on at least one side with a conductive material, such as ametal or metal alloy.

[0098] The contact pad 3 is the terminal end of a communication linewhich electrically communicates with an electronic device formed on thesubstrate 14, such as a transistor, a display electrode, or otherelectrical device. The contact pad 3 is typically made of aluminum, butcan be made of any conductive material. The release layer 13 is made ofsilicon nitride, Si, Ti or other etchable material. However, the releaselayer 13 is not necessary and can be eliminated. The release layer 13and the contact pad 3 are formed on or over a substrate 14, which isformed of a material, such as oxidized silicon or glass or a printedcircuit board or ceramic or silicon or gallium arsenide.

[0099] As shown in FIG. 7, a strip of metal having no stress gradientinherent in the metal will lie flat. However, as shown in FIG. 8, when auniform stress gradient is introduced into the strip of metal, the stripbends into an arc.

[0100] Each spring contact 15 is formed such that the stress gradientshown in FIG. 8 is introduced into the spring contact 15. When thespring contact 15 is formed, the metal layer comprising the springcontact 15 is deposited such that compressive stress is present in lowerportions of the metal layer and tensile stress is present in upperportions of the metal layer. FIG. 8 shows the stress difference Δσ(i.e., the difference in stress from the top to the bottom) present inthe spring contact 15. Compressive stress in lower portions of the metallayer is depicted by arrows directed to the left. Tensile stress isdepicted in upper portions of the metal layer by arrows directed to theright. The stress gradient (stress difference divided by thickness)causes the spring contact 15 to bend into the shape of an arc having aradius r. Equation 1 gives the radius of curvature r of the springcontact 15: $\begin{matrix}{r = {\left( \frac{y}{1 - v} \right)\frac{h}{\Delta \quad \sigma}}} & (1)\end{matrix}$

[0101] where y is the Young's modulus of the metal, h is the thicknessof the metal layer forming the spring contact 15, Δσ is the stressdifference, and v is the shear modulus of the metal.

[0102] Referring again to FIG. 6, r is the radius of curvature of thefree portion 11 of the spring contact 15 as predicted in Equation 1, andθ is the angle separating the radius line directed toward the junctionof the free portion 11 with the anchor portion 12 and the radius linedirected toward the tip 30 of the free portion 11. Equation 2 gives theapproximate height b of the spring contact tip 30 from the substrate 14for angles θ<50°: $\begin{matrix}{b \approx \frac{L^{2}}{2r}} & (2)\end{matrix}$

[0103] where L is the length of the free portion 11 and r is the radiusof curvature of the free portion 11.

[0104] Since each spring contact 15 is preferably made of a highlyelastic material, each spring contact 15 can be pushed down at the tip30 and deformed as shown in FIG. 6, but will not plastically deform.Typically, a contact pad 3 of a device 101 exerts the downward forceplaced on the tip 30 and electrically contacts the tip 30. The springcontact 15 resists the downward force placed on the tip 30 and maintainselectrical contact with the contact pad 3.

[0105] When the force on the tip 30 is released, the spring contact willreturn to its undeformed state. Thus, the elasticity of the springcontacts 15 allows the spring contacts 15 to make numerous electricalconnections with different contact pads 3 while maintaining theintegrity of the electrical connection between the spring contact tip 30and the contact pads 3.

[0106] Additionally, the spring contact 15 is preferably made of a creepresistant material. Therefore, when the spring contact 15 is elasticallydeformed over an extended period by a contact pad 3 pressing down on thespring contact tip 30, the spring contact 15 resists the downward forceand pushes the spring contact tip 30 against the contact pad 3,maintaining the electrical connection.

[0107]FIG. 9 shows a model for determining the amount of force F_(tip)applied by the spring contact tip 30 to a contact pad 3 in reaction tothe force of the contact pad 3 pressing down on the spring contact tip30. Equation 3 gives the reaction force F_(tip) of the spring contacttip 30: $\begin{matrix}{F_{tip} = \frac{{wh}^{2}\Delta \quad \sigma}{12x}} & (3)\end{matrix}$

[0108] where w is the width of the spring contact 15, h is the thicknessof the spring contact 15, Δσis the stress gradient and x is thehorizontal distance from the spring contact tip 30 to the point wherethe spring contact 15 first touches the substrate 14.

[0109] For a given width w, thickness h and stress gradient Δσ, thereaction force F_(tip) of the tip 30 varies inversely with the distancex. Therefore, the reaction force F_(tip) increases as the spring contacttip 30 gets closer to the substrate 14, since the distance x decreasesas the spring contact 15 collapses and presses against the substrate 14as shown in FIG. 6. The increase in the reaction force F_(tip) as thecontact pad 3 presses the spring contact tip 30 closer to the substrate14 generally improves the electrical connection between the springcontact tip 30 and the contact pad 3. The increasing reaction forceF_(tip) causes the spring contact tip 30 to deform locally at thecontact pad 3, increasing the area of contact between the contact pad 3and the spring contact tip 30.

[0110] FIGS. 10-13 show the basic steps in one method for forming aspring contact 15. In FIG. 10, a contact pad 3 is formed on or over asubstrate 14. Additionally, an release layer 13 is formed on or over thesubstrate 14. However, as mentioned above, the release layer 13 is notrequired and can be eliminated.

[0111] In FIG. 11, a layer of metal 16 is deposited on or over thesubstrate 14. In the preferred embodiment of the invention, the metal isthe nickel-zirconium alloy described above. Part of the metal layer 16is electrically connected to or directly contacts the contact pad 3 andanother portion of the metal layer 16 is deposited on or over therelease layer 13. There are many methods available for depositing ametal layer 16 on or over the substrate 14, including electron-beamdeposition, molecular beam epitaxy, chemical vapor deposition andsputter deposition. Preferably, the metal layer 16 is sputter deposited.

[0112] When sputter depositing a metal, the metal to be deposited isplaced on a target and set to a high negative voltage. A stream ofplasma gas, typically argon, is directed towards the target. The highvoltage potential between the plasma gas and the target metal producesions which are accelerated toward and bombard the metal target. Thisbombardment knocks small particles of the metal target free and thefreed particles are guided to the surface where the particles aredeposited.

[0113] The metal layer 16 is deposited in several sub-layers 16-1 to16-n to a final thickness h of approximately 1 μm. The stress differenceΔσ is introduced into the metal layer 16 by altering the stress inherentin each of the sub-layers 16-1 to 16-n of the metal layer 16, as shownin FIG. 11, each sub-layer 16-x having a different level of inherentstress.

[0114] Different stress levels can be introduced into each sub-layer16-x of the deposited metal layer 16 during sputter deposition in avariety of ways, including adding a reactive gas to the plasma,depositing the metal at an angle, or varying the deposition angle, andchanging the pressure of the plasma gas. Preferably, the differentlevels of stress are introduced into the metal layer 16 by varying thepressure of the plasma gas, which is preferably argon.

[0115]FIG. 14 is a graph showing the relationship of the film stress inthe sputter deposited nickel-zirconium alloy and the pressure of theplasma gas used in the deposition. For low pressures of the plasma gas,approximately 1 mTorr, the film stress in the deposited metal iscompressive. As the pressure of the plasma gas increases, the filmstress in the deposited sub-layer changes to a tensile stress andincreases with increasing plasma gas pressure.

[0116] Preferably, the metal layer 16 is deposited in five sub-layers16-1 to 16-5. The first sub-layer 16-1 is deposited at a plasma gaspressure of 1 mTorr, as indicated by numeral 1 in FIG. 14. The firstsub-layer 16-1 is the bottom-most layer in the metal layer 16 and has aninherent compressive stress. The second sub-layer 16-2 is deposited ontop of the first sub-layer 16-1 at a plasma gas pressure ofapproximately 6 mTorr. The second sub-layer 16-2 has a slight inherenttensile stress, as indicated by numeral 2 in FIG. 14. Sub-layers 16-3,16-4 and 16-5 are then deposited one on top of the other at the plasmagas pressures indicated by numerals 3, 4 and 5 in FIG. 14.

[0117] The process of depositing the metal layer 16 in five separatesub-layers 16-1 to 16-5 results in the metal layer 16 having a stressdifference Δσ which is compressive in the lower portion of the metallayer 16 and becomes increasingly tensile toward the top of the metallayer 16. Although the stress gradient urges the metal layer 16 to bendinto an arc, the metal layer 16 adheres to the release layer 13, thesubstrate 14 and the contact pad 3 and thus lies flat.

[0118] After the metal layer 16 is deposited, the metal layer 16 isphotolithographically patterned into the spring contacts 15.Photolithographic patterning is a well-known technique and is routinelyused in the semiconductor chip industry. Photolithographicallypatterning the metal layer 16 is completed generally as shown in FIGS.11-13. A photosensitive material 17 is evenly deposited on the topsurface of the metal layer 16. The photosensitive layer 17 is thensoft-baked at a temperature of approximately 120° F. The photosensitivelayer 17 is then exposed to light, typically in the ultra-violetspectrum, using an appropriate mask. The mask ensures that areas of thephotosensitive material 17 are appropriately exposed to the light whichdescribes a two-dimensional view of the spring contacts 15.

[0119] Once the photosensitive material 17 is exposed to the appropriatepattern of light, the photosensitive material 17 is developed andhard-baked at a temperature of approximately 200° F. The elasticmaterial 16 is then etched to form the spring contacts 15. Differentmethods for etching can be used, including ion milling, reactive ionetching, plasma etching and wet chemical etching. Preferably, wetchemical etching is used.

[0120] The wet chemical etchant, for example, a nitric acid solution, isapplied to the elastic material 16. The etchant removes appropriateareas of the photosensitive material 17, determined by which areas ofthe photosensitive material 17 were exposed or not exposed to the lightand the type of photosensitive material 17 used. When the appropriateareas of photosensitive material 17 are removed, the etchant removes theareas of the metal layer 16 lying under the removed areas ofphotosensitive material 17. The remaining areas of the metal layer 16form the spring contacts 15. A top-view of one spring contact 15 isshown in FIG. 15. The area of the metal layer 16 removed by the etchantis described by the dashed line 18.

[0121] Next, as shown in FIG. 12, the free portion 11 of the springcontact 15 is released from the release layer 13 by a process ofunder-cut etching. Until the free portion 11 is released from therelease layer 13, the free portion 11 adheres to the release layer 13and the spring contact 15 lies flat on the substrate 14. A second layerof the photosensitive material 17 is deposited on top of the springcontacts 15 and on the area surrounding the spring contacts 15. Thesecond layer of the photosensitive material 17 is then exposed to lightusing the appropriate mask, developed and hard-baked. A selectiveetchant is then applied to the photosensitive material 17 and removesareas of the photosensitive material 17 around the spring contacts 15.The etchant is called a selective etchant because after the areas ofphotosensitive material 17 around the spring contacts 15 are removed,the etchant proceeds to etch the release layer 13 underneath the springcontacts 15. The photosensitive material 17 on top of the springcontacts 15 resists the selective etchant and protects the springcontacts 15. The selective etchant etches the release layer 13 fasterthan the selective etchant removes metal from the spring contacts 15.This means that the spring contacts 15 are released from the releaselayer 13 and are allowed to bend up and away from the release layer 13due to the stress gradient in the spring contacts 15.

[0122] Only those areas of the release layer 13 under the free portion11 of the spring contact 15 are under-cut etched. The area of releaselayer 13 under-cut etched for each spring contact 15 is described by theshaded portion in FIG. 17. This means that the anchor portion 12 of thespring contact 15 remains fixed to the release layer 13 and does notpull away from the release layer 13. It should be appreciated that themethod for patterning the metal layer 16 onto the spring contacts 15should not result in any annealing of the metal layer 16.

[0123] Once the free portion 11 is freed from the release layer 13, thestress gradient causes the free portion 11 to bend up and away from thesubstrate 14. The stress gradient is still inherent in the anchorportion 12 and urges the anchor portion 12 to pull away from thesubstrate 14.

[0124] To decrease the chance of the anchor portion 12 pulling away fromthe substrate 14, the spring contact 15 can be annealed to relieve thestress in the anchor portion 12. This annealing process does not affectthe free portion 11 because, once the free portion 11 is released andallowed to bend up, no stress remains on the free portion 11 to berelieved by annealing. Thus, the stress gradient remains in the freeportion 11, and the free portion 11 remains curved up and away from thesubstrate 14 after annealing.

[0125] Finally, FIG. 13 shows a layer of gold 19 plated over the outersurface of each spring contact 15. The layer of gold 19 is preferablyused to reduce the resistance in the spring contacts 15, but can bereplaced with any other conductive material. Preferably, the gold layer19 is plated on the spring contacts 15 using a plating process.

[0126] Additional steps can be added to the under-cut etching process toimprove the process if necessary. For example, etchant vias, or smallwindows, can be etched into the free portions 11 of the spring contacts15. The etchant vias operate to provide the selective etchant fasteraccess to the release layer 13, thereby speeding the process ofreleasing the free portions 11 from the release layer 13. Also, a hardmask can be applied to the top surface of the spring contacts 15 toensure that the selective etchant does not remove material from the topsurface of the spring contacts 15 in case the photosensitive material 17protecting the top of the spring contacts 15 fails.

[0127] Since the process for forming the spring contacts 15 is limitedonly by the design rules of photolithographic patterning, many hundredsor thousands of spring contacts 15 can be formed closely together in arelatively small area on the substrate 14. The typical width w of thespring contact 15 is 40-60 μm. Therefore, the spring contacts 15 can beformed close together, at a spacing of approximately 10-20 μm. Thismakes the center-to-center distance between adjacent spring contacts 15approximately 50-80 μm, which is well within the typicalcenter-to-center distance between adjacent contact pads 3 on a standardsemiconductor chip 2.

[0128] To test the effectiveness of the spring contacts 15 inapplications similar to those found in solder-bump flip-chip bonding, atest array of the spring contacts 15 at a center-to-center spacing of 80mm was developed as shown in FIG. 16. Four sets of arrays 20 of thespring contacts 15 were formed on a bottom substrate 21. Fourcorresponding arrays of linked contact pads 22 were formed on an uppersubstrate 23. The upper substrate 23 and the lower substrate 21 werebrought together such that the spring contacts 15 contacted acorresponding contact pad 3. The resistance R was then measured acrosspairs of the spring contact 15 leads.

[0129]FIG. 17 graphically depicts the measured resistance R for eachspring contact pair in the test apparatus. The measured resistance Rwithin each array generally trends upward from left to right because ofthe increased conductor length of the spring contacts 15 positioned tothe right compared to the spring contacts 15 positioned to the left ineach array. Most of the approximately 25-30 ohms of resistance measuredfor each spring contact 15 pair is due to the length and geometry of theconductors extending between the spring contacts 15 and the contact pads3.

[0130]FIG. 18 shows the total resistance of the connection between aspring contact 15 and corresponding contact pad 3. As shown in FIG. 18,approximately 1.5 ohms of resistance is due to the conductors leading tothe contact pad 3 and the spring contact 15. Approximately 0.2 ohms ofresistance is due to the shape of the spring contact tip 30. Theremaining resistance, approximately 0.5-0.8 ohms, is the resistance atthe interface between the contact pad 3 and the spring contact tip 30.

[0131] In general, the resistance at the interface between the contactpad 3 and the spring contact tip 30 decreases as the height b decreases.As mentioned above, the reaction force F_(tip) that the spring contacttip 30 exerts against the contact pad 3 increases as the contact pad 3pushes the spring contact tip 30 closer to the substrate 14. Theincreased reaction force F_(tip) causes the spring contact tip 30 tolocally deform at the contact pad 3, thereby increasing the contact areaand decreasing the resistance at the interface.

[0132] The shape of the spring contact tip 30 takes different formsdepending on the application. FIGS. 19-24 show a series of six differenttip 30 shapes tested. Although only four of each type of spring contacttip 30 were tested, none of the spring contact tip 30 shapes showed asignificant superiority over any other type of spring contact tip 30.

[0133] As mentioned above, since the production of the spring contacts15 is limited only by the design rules of photolithographic patterning,the spring contacts 15 can be used to interconnect numerous differenttypes of devices. For example, FIGS. 25-26 show the substrate 14 havinga plurality of spring contacts 15 formed on the top surface of thesubstrate 14. The contact pads 3 formed on the lower surface of the chip2 are electrically connected to corresponding spring contacts 15 on thesubstrate 14. An adhesive 24 holds the chip 2 stationary relative to thesubstrate 14. A dust cover, or can, 25 covers the chip 2 and ishermetically sealed to the substrate 14. The dust cover 25 assures thatmoisture and other foreign substances do not corrode the spring contacts15 or the contact pads 3, or otherwise interfere with the electricalconnections between the individual spring contacts 15 and thecorresponding contact pads 3.

[0134]FIG. 28 shows an alternate embodiment of a connecting device forelectrically connecting two devices. A wafer 26 is shown having aplurality of spring contacts 15 formed on opposite sides of the wafer.Pairs of the spring contacts 15 on opposite sides of the wafer 26communicate with each other and electrically connect the contact pads 3on both the chip 2 and the substrate 14. This embodiment of theinvention allows processing of the chip 2 and the substrate 14 withoutrisking damage to the spring contacts 15. The wafer 26 is used tointerconnect the chip 2 and the substrate 14 only after all processingis completed on the chip 2 and the substrate 14.

[0135]FIG. 27 shows another embodiment of the invention. The springcontacts 15 are formed on the lower surface of the chip 2. The springcontacts 15 contact corresponding contact pads 3 on the substrate 14.The adhesive 24 holds the chip 2 stationary with respect to substrate14.

[0136] The spring contacts 15 are not limited to interconnecting a chip2 to a substrate 14 or circuit board. The spring contacts 15 are usedequally well to interconnect two chips 2, two circuit boards, or otherelectronic devices to each other. One such alternative use for thespring contacts 15 is in probe cards. As discussed above, probe cards 7are used to temporarily connect two devices, typically when one of thedevices is tested. Such testing is common in the semiconductor industry,where the probe cards 7 are used to test semiconductor chips while thechips are still part of a single-crystal silicon wafer.

[0137]FIG. 29 shows an embodiment of the invention where the probe card27 has an array of spring contacts 15 used in place of the standardprobe needles 8. The probe card 27 operates identically to the standardprobe card 7 except for having spring contacts 15. The probe card 27 isaligned with the device 10 such that the spring contacts 15 compliantlycontact the corresponding contact pads 3 on the device 10. The device 10is then tested or communicated with by a testing device electricallyconnected to the probe card 27.

[0138] An example testing device is shown in FIG. 30. A display patterngenerator 40 communicates with driver chips 42 mounted on the twofull-width probe cards 27. The probe cards 27 have the spring contacts15 which contact associated addressing lines 43 formed on the displayplate 44. The addressing lines 43 communicate with display electrodes(not shown). Therefore, the display pattern generator 40 can drive thedisplay electrodes to produce a matrix of electric potentialscorresponding to a test image. Sensors (not shown) on the sensor plate45 detect the matrix of electric potentials on the display electrodesand generate signals each corresponding to the electric potential. Thesignals are read out by scanner chips 46 mounted on the sensor plate 45.The test signal analyzer 41 receives the signals from the scanner chips46 and forms a sensed image corresponding to the signals. The testsignal analyzer 41 then compares the sensed image with the test imageoutput by the display pattern generator 40 to determine if the displayplate 44 and display electrodes are working properly.

[0139] Since producing a standard probe card 7 having probe needles 8 islabor intensive and time-consuming, standard probe cards 7 are notgenerally made to contact all of the addressing lines 43 on the displayplate 44. Therefore, testing of the display plate 44 must be done insections since the probe cards 7 cannot accommodate the full width ofthe addressing lines 43. In contrast, the probe card 27 made with springcontacts 15 can be made easily and inexpensively. Also, the probe cards27 having the spring contacts 15 can be made to any width and thereforecan test all of the data or address lines of an apparatus, such as thedisplay shown in FIG. 26, at one time.

[0140] The foregoing techniques for the manufacture of springs for probecards and other applications can be extended to the manufacture of coilstructures. Springs are made by introducing an intrinsic stress profileof a certain amount designed to produce the desired spring height andcurvature. Similarly, a reproducible built-in stress gradient orintrinsic stress profile can be designed into a thin film by varying thegrowth conditions appropriately during deposition to produce coilstructures, i.e. a spring which bends back on itself producing a loopwinding and contacting the substrate. By adding one or more conductivelayers, a coil structure suitable for use as an inductor may bemanufactured.

[0141] The intrinsic stress of many sputtered thin films depends on theambient pressure at which the material is deposited. By varying thepressure during sputtering, films can be obtained that are compressivelystressed near the substrate-film interface and tensile stressed at thefilm surface. FIG. 32 shows such a stress-graded film 104 sandwichedbetween two gold layers 102, 106. The stress graded film can be NiZr,Mo/Cr, solder-wettable Ni, or other suitable material. The bottom goldlayer 106 forms the outer skin of the coil when released and provides ahigh conductivity path for electrons at high frequencies. The top goldlayer passivates the surface. The metal stack is deposited above asuitable release layer 108 such as Ti, Si, or SiN. The release layershould be a material that can be quickly removed by selective dry or wetundercut etching. Possible etchants for a Si release layer include KOH(wet processing) and XeF₂ (dry processing).

[0142] In FIG. 33, a released structure with continuous layers 106 and104 is shown. The challenge of connecting the free end of the loop to acontact pad on the same substrate is made difficult by the fact that theloop typically has a constant radius of curvature, and therefore thefree end will naturally return to the take-off point. Several techniquescan be used to resolve this problem as described below.

[0143] The scanning electron micrograph in FIG. 34 shows a series ofout-of-plane micro-inductor windings produced according to theinvention. The coil windings were made using stress engineered thinfilms that are deposited by sputtering. The film isphotolithographically patterned into strips of micro-springs or elasticmembers that are subsequently released from their underlying substrate.Upon release, the built-in stress gradient causes the elastic members tocurl and form three-dimensional out-of-plane loops that make up theinductor coil. In the coil shown in FIG. 34, each loop has just enoughhelical pitch for each free end to contact the adjacent pad of thearray. The helical twisting provides the useful feature that the freeend of the elastic member is shifted longitudinally (or axially) fromthe take-off point. This allows for the formation of a continuousinductor consisting of multiple turns without interruption of the springmetal. To protect the inductor in actual use on a chip or circuit board,the loops can be enclosed in a molding compound.

[0144] In the specific example of FIG. 34, the stress graded metal is a0.3 μm-thick 85 Mo/15 Cr alloy deposited at five progressivelyincreasing pressures. The film was patterned into 4 μm wide elasticmembers that were then released by removing an underlying PECVD SiNlayer using 10:1 buffered HF. The released elastic members formed 70 μmdiameter circular loops. After rinsing in D.I. water, the releasedelastic members were pressed against a flat surface and the substratewas heated to 85° C. The compression holds the springs tightly whilewater is slowly evaporated. This technique prevents liquid surfacetension from pulling adjacent elastic members into a tangled mess aswater evaporates. For many applications, wider and thicker elasticmembers forming bigger loops are desired. These larger coils are easierto make than those in FIG. 34 because less stress gradient is required.Furthermore, wider springs are stiffer and generally less susceptible togetting entangled with adjacent members during spring release.

[0145]FIG. 35 shows some of the process steps for forming a multi-turncoil structure from a series of individual circular coil structures.Initially, a plurality of elastic members 61 a-65 a are patterned over arelease window. Each elastic member 61 a-65 a is part of a largerpatterned structure 61-65. For example, structure 61 includes elasticmember 61 a, connecting pad 61 b and contact pad 61 c. To form acontinuous structure, each loop must be electrically connected to thenext adjacent loop. After remove of release window 66, each elasticmember 61 a-65 a coils back on itself. When released, the elasticmembers 61 a-65 a will form circular loops with radiuses given byequation (1) above. The length of each of 61 a-65 a is designed so thatthe elastic members do not span complete loops when released. The tips(free ends) are left hanging just shy of an opposite contact, which isthe contact pad 62 c-65 c of the adjacent loop. The loops are thenpressed down on the contact and soldered or plated together. Theresultant multi-turn coil structure, with coil axis 68 begins at 61 a-61b, then the first loop winding 61 c, which is connected to contact pad62 a, and so on.

[0146]FIGS. 36 and 37 show an alternative approach for forming the coilconnections. In this approach, a mechanical barrier or stop 71 is fixedto the substrate at the end of contact pad 62 c, in order to receive thetip of elastic member 61 c. This approach uses almost full-lengthelastic members aided by a mechanical barrier 71. It is important todesign the dimensions of the mechanical stop properly and to positionthe stop correctly so that the structure lies entirely within thetrajectory of the tip as illustrated in FIG. 37. Otherwise, the elasticmembers may be caught on the near edge of the stop during release. InFIG. 37, the dashed line shows the tip trajectory.

[0147]FIG. 38 shows a graph of tip trajectories for 200 μm diametercoils having different elastic member lengths. The parameter in thefigure, i, corresponds to length of the elastic member in multiples ofπr/4, where r is the coil radius. The x=0 point in the figure is theedge of the release window. It is interesting to note that the tip ofthe full length spring, i=8, stays on the right of the release edgethroughout its entire trajectory. Since the mechanical block has to beplaced at x<0, the length of the elastic member needs to be made lessthan a full circumference. The range of possible trajectories also placeconstraints on the dimensions of the mechanical block.

[0148] In addition to the mechanical stop, another method forpositioning the free tip tangentially, away from the takeoff pointincludes varying the radius of curvature of the elastic member. If theradius of curvature if varied along the length of the elastic member, agenerally circular coil will be formed. An unequal radius of curvaturewill cause the free tip to stop at some point away from the takeoffpoint. If the radius of curvature varies as a function of length andwidth of the elastic member, a helical coil will be formed. The radiusof curvature of the elastic member may be varied, for example, by addinga load layer uniformly across the width of one or more portions of theelastic member. The radius of curvature may also be varied by patterningone or more openings or perforations uniformly across the width of aportion of the elastic member. Some combination of load layer andperforations (or openings) may also be used. Perforations and loadlayers may also be used to create helical windings as described below.

[0149]FIG. 39 shows another approach for forming a multi-turn coil withindividual loop windings. In this embodiment, release window 66 isdefined to have a skewed angle relative to the run length of eachelastic member 61 c-64 c. When the elastic member is released, the coilloops lean sideways contacting the adjacent contact pad. Thus loopwinding 61 c contacts pad 62 a. This lateral bending can also be inducedby designing a built-in stress anisotropy in the springs (describedbelow). When the springs are pressed down, their tips slide over toneighboring contact pads and a continuous coil is formed.

[0150] To take full advantage of the available conducting path, the coilthickness, h, should be made at least as thick as the skin depth δ:$\begin{matrix}{{h \geq \delta} = \sqrt{\frac{\rho}{\pi \quad \mu \quad f}}} & (4)\end{matrix}$

[0151] where ρ is the resistivity of the coil conductor, μ is itsmagnetic permeability, and f is the operating frequency. Making the filmthicker than the skin depth does not improve film conductance becausemost of the current is confined to within the skin depth of theconductor surface. For frequencies of interest (around 1 GHz), the idealfilm thickness is between 1 μm to 3 μm, a thickness range that iscompatible with current deposition and patterning processes.

[0152] If the coil material is composed of one elastic material with astress gradient, with the film thickness determined, the coil loopradius can be calculated using equation 1. If there are additionallayers, the stress profile is not a linear gradient and equation 1 needsto be modified. The spring length, l, should then be designed to about

l=2πr  (5)

[0153] for the elastic members to form complete circular loops whenreleased. The number of coil turns, N, is next determined based on adesired inductance which approximately equals: $\begin{matrix}{L = {\mu_{0}\frac{N\quad \pi \quad r^{3}}{x}}} & (6)\end{matrix}$

[0154] where x is the pitch between coil windings and μ₀ is thepermeability of air (for air-cored coils). While equation 6 is good fortoroids and long solenoid (N*x>>r), more complicated expressions forshort solenoids are available from textbooks. The spring width, w, canbe made as wide as necessary to accommodate an acceptable electricalresistance, R, through the following approximation: $\begin{matrix}{w = \frac{2\quad \pi \quad r\quad \rho \quad N}{\delta \quad R}} & (7)\end{matrix}$

[0155] Equations 6 and 7 indicate that there is a tradeoff betweeninductance and resistance. Wide elastic members, small number of loops,and a short radius produce low resistance but also lead to lowinductance. The ratio between coil impedance and resistance, also calledthe coil quality factor Q, is a good parameter for assessing how lossesinfluence coil performance: $\begin{matrix}{Q = \frac{2\quad \pi \quad {fL}}{R}} & (8)\end{matrix}$

[0156] This dimensionless parameter determines the sharpness of theresonance peaks of LC resonators, the selectivity of LC filters, theamount of oscillator jitter, and the gain of resonant amplifiers.Looking now again at equations 6 and 7, it can be seen that the qualityfactor increases with coil diameter and with the ratio between conductorwidth to the winding pitch: $\begin{matrix}{Q \approx {\frac{\pi \quad f\quad \mu_{0}}{\rho/\delta}\frac{w}{x}r}} & (9)\end{matrix}$

[0157] Also, the importance of a low AC sheet resistance, ρ/δ, isexplicitly expressed in equation 9.

[0158] Table 1 tabulates a few representative inductance values and Qfactors for out-of-plane coils produced in accordance with theinvention. A conductor resistivity of 2.5 μΩ-cm is assumed in theestimates. The quality factors are roughly approximated by supposingthat the current flows as a uniform sheet with a sheet thickness equalto the skin depth. The actual quality factor may be up to a factor of 2smaller due to proximity effect that is not included in thesecalculations. The listed Q numbers should be compared to the best valuesof 10 to 20 that are currently obtained with state-of-the-art in-planecoils utilizing high aspect ratio windings and removed substrate. TABLE1 Matrix of Values for Typical Air-Cored Out-of-Plane Inductor CoilsCoil Coil Line width Line pitch diameter length L Q @ [μm] [μm] [μm][μm] # turns [nH] 1 GHz 4 8 70 76 10 6.4 7.3 4 8 70 796 100 61 7.0 24 30200 294 10 13 32 24 30 200 2994 100 132 32 54 60 500 594 10 41 90 54 60500 5994 100 412 90 90 100 1000 990 10 100 180 90 100 1000 9990 100 988180

[0159] In addition to the “diagonal” release window for connectingindividual loop windings to one another as described in FIG. 35, manyother types of connections are possible. An alternate embodiment isshown in FIG. 40 which utilizes symmetric wedge take-off points. In FIG.40, elastic members 81 a-85 a are deposited and patterned on asubstrate. Each elastic member, for example, 81 a, includes a patternedcontact pad arrangement. This contact pad arrangement includes aU-shaped portion 81 b, which includes two tip portions, 81 c and 81 d.Also included for support is symmetric element 81 e. The symmetricsupports balance opposing biaxial stresses in the released film 81 a inorder to reduce lateral bending of the coil windings. Placing therelease point lower than a mating contact pad also brings the elasticmember tip to the proper contact point without mechanical blocks. Thisdesign alternative enables better contacting at the expense of aslightly longer winding pitch. When elastic members 81 a-85 a arereleased, they coil and contact pad portions 82 c-86 c(not shown).

[0160] The multi-turn coil designs in FIGS. 35, 36, 39 and 40 providelinear coil arrangements, i.e., the coil axis is a straight line. Eachof these designs can also be arranged in a circular layout to formmicro-toroids, i.e., the coil axis is in a circle. A micro-spring toroidis shown in FIG. 41, with coil axis 91 and each coil turn 92 shownschematically. Toroids are attractive because they confine magneticfields very tightly within their windings, thus allowing multiple coilsto be packed very closely without mutual coupling. The absence of straymagnetic fields also further reduces lossy substrate eddy currents.

[0161] Unlike in-plane coils, the individual windings of out-of-planecoils are easily accessible at arbitrary locations along the inductor.Therefore, it is possible to obtain different inductances from a singlecoil by tapping the windings at appropriate locations. When combinedwith transistor switches, these taps can be used to make variableinductors useful in tunable filters and resonators. FIG. 42 shows howthe coil of FIG. 41 can be modified by adding tap 93 at contact pad 61a, tap 94 at contact pad 62 a and tap 95 at contact pad 65 a. Note thatthe tap points depend on N, the number of windings between taps. Betweentaps 93 and 94, N=1 and between taps 93 and 95, N=4.

[0162] In addition to their use as inductors, the out-of-plane coils canbe used as transformers. Micro-transformers are essential in electroniccomponents such as mixers, double-tuned filters and RF signaltransformers. The out-of-plane coils are compatible with a variety ofmicro-transformer architectures. FIG. 43 shows an embodiment in the formof a toroidal transformer with an air core, which includes primarywinding 124 having input/output 122 and secondary winding 126 withinput/output 120. The voltage relationship between the two coupled coilsis determined by the ratio of turns between the primary and secondarywindings. The pairs of arrows 120 and 122 indicate current paths intoand out of the two windings, 124 and 126.

[0163]FIG. 44 shows an alternate design for an air-core transformer withintertwined primary 124 and secondary 126 windings. The multipleoutbound arrows 127 in the secondary coil 126 illustrate the possibilityof coil tapping for obtaining variable transformer ratios. The insetshows the micro-spring layout necessary for implementing the transformerarchitecture. Coil tapping is, naturally, also compatible with thedevice in FIG. 43.

[0164] Ferromagnetic cores are attractive for many coil applicationsbecause of their ability to increase coil inductance and to channel andconfine magnetic fields to well defined regions. For high frequency GHzapplications, however, any ferromagnetic material used has to beelectrically insulating. Otherwise, excessive loss leading to low Q willresult.

[0165] The micro-coils can be embedded in an epoxy matrix that containsferrite particles, after they are released from the substrate. Thiscreates a ferrite core in and around the micro-coil that increases thecoil inductance. It is also the method of choice to confine the magneticfields of solenoids. The field lines outside the solenoid do not fan outanymore because the ferrite around the coil closes the magnetic pathway.

[0166] Coils are magnetically isolated from each other by using anisland of ferromagnetic material for each individual coil. Coil windingsare therefore placed in deep pockets made by patterning spin-coated BCBor another thick film. After release of the elastic member, the pocketis filled with ferromagnetic particles of a suitable size immersed in aninsulating epoxy matrix.

[0167] Another approach utilizes ferromagnetic metal cores that can bedeposited and patterned in ways that are compatible withmicrofabrication. However, since these cores are conductive, theirapplications are limited to lower frequencies. FIG. 52 shows such adevice employing an electroplated permalloy (NiFe) core. In thisembodiment, a layer of SiN_(x) 202 is deposited on the substrate 200,followed by the elastic member 204. A thick film 206, such as SU-8photoresist, is first patterned to define a window for plating the corematerial. The NiFe core 208 is plated above a thin vacuum-deposited seedlayer which in turn lies on top of an insulating dielectric 210. TheSU-8 layer 206 is then removed, followed by release of elastic member204 to form loops that enclose the core. The coil losses can be reducedto some extend by laminating the core 208 as shown in FIG. 52.

[0168]FIGS. 45 and 46 are not drawn using actual scale and aspectratios. In particular, the core 208 has to be designed so that itconforms to the constraints discussed in FIG. 38. This constraint makesthe core occupy much less than the available cross sectional area of thecoil. However, for typical core relative permeability of about 1000,even a 10% fill factor will increase the inductance of an air-coredevice by about 100-fold.

[0169] A metallic or ceramic ferromagnetic core can also be formed byphysically attaching a pre-made core 206 on the substrate 200 prior torelease of the elastic members 220 as shown in FIG. 47. The placementcan be performed by an automatic pick and place equipment commonly usedin the chip industry. The dimensions of the pre-made core will,naturally, also have to conform to the same constrains discussed in FIG.38.

[0170]FIG. 47 illustrates how a ferromagnetic core micro-transformer canbe fabricated using the methods described above. FIG. 47a shows theelastic member 220 layout prior to release. Two sets of metal linesfacing opposite each other for the primary and secondary windings areplaced within the BCB pocket. After release of the elastic members 220,the pocket is filled with a ferromagnetic epoxy. An illustration of thereleased elastic members is shown in FIG. 47b. The loop 224 in FIG. 47btraces a magnetic path coupling the primary and secondary coils. Thepocket is designed with features that extend towards the coil axismidway down each coil. These features are meant to obstruct stray fieldsand improve the intended coupling between the primary and secondarywindings. Although the estimated coupling for the transformer in FIG. 47is only about 66%, significant improvements are possible ifphoto-definable filling materials are used.

[0171] An alternative ferromagnetic core micro-transformer can befabricated from the methods discussed in FIGS. 45 and 46. In thisembodiment, the core of FIGS. 51 and 52 is fashioned in a loop thatmagnetically couples a set of two or more coil windings. To reduce thepossibility of core saturation, a small air gap can be placed to breakthe core loop.

[0172] The above described coil structures have circular loop windings.Such coil structures can also be manufactured using coils with a helicaltwist.

[0173] It has been observed that a helical twist develops in somereleased structures. The origin of this twist is stress anisotropy.Specifically, in a planetary deposition system, the radial andtangential components of the stress in the film vary at different rates,producing stresses of differing magnitude. The stress anisotropy givesrise to a radial-tangential shear. The pressure in the sputter system isvaried during deposition to produce a stress gradient, however, becausethe stress is anisotropic, a shear gradient develops as well. Thisapplies a torque to the spring, giving it a finite helical pitch. Thehelical pitch causes the tip of the released spring to move off the axisof the spring.

[0174] It has also been observed that wider finger structures tend tolift more than narrow finger structures of the same thickness. Springscan twist in only one direction at a time, and so cannot relax stresscompletely in one direction. As plane strain conditions exist near thelongitudinal centerline of wider springs, intrinsic longitudinalstresses relax completely, while transverse stresses can only relax verynear the edges.

[0175]FIG. 48 illustrates how different helical pitch results fromvaried spring orientation. The springs were made from metal deposited ina planetary sputter system.

[0176] The planetary motion of the wafer in the vacuum system producesgeometric differences in the flux arrival in the radial and tangentialdirections of the wafer. This causes the stress in the radial andtangential directions of the wafer to be unequal. Two loops are shown,the one to the left 130 is oriented along a direction of principlestress, and as a result, the helical bending is practically zero. Thespring on the right 132 in FIG. 48 is oriented at 45 degrees to theprinciple axis, and as a result has a large helical pitch, on the orderof the loop diameter. Therefore, by taking a metal film of known stressanisotropy, in this case about 8.6%, and by orienting the spring at thedesired angle to the principle axis, the pitch may be usefullycontrolled.

[0177] Inventor David Fork's co-pending application D/A0505 (IP/A00002)which is filed the same day as this application and is incorporatedherein by reference discloses manufacturing methods for sputtering thinfilms with controlled stress anisotropy. Other methods for creating ahelical twist in the loop winding are disclosed below.

[0178]FIG. 49 illustrates how a multi-turn coil 140 utilizing singlehelical turns 142 might be configured on a substrate. Each turn of thecoil has a sufficient degree of helical pitch to jog the free end of theloop over to a contact pad 144 adjacent to the loop 142. The free end inmaking mechanical contact with the pad may also make electrical contact.Robust electrical and mechanical contact can be improved by for examplesoldering the free end of the loop to the pad 143. The illustration inFIG. 49 shows a jog between the end of the first loop and the base ofthe adjacent loop. This is done for clarity, and is not necessary forthe actual device. The performance is better for coils with denserwindings, therefore it is advantageous to pack the coils as tightly aspossible.

[0179]FIG. 50 shows a multi-turn coil 150. For such a coil, the springmetal is patterned into a strip that is long enough to span the entirecoil. The illustration shows 4 turns. In principle, the number of turnsis limited by the length of the substrate, since the length of thespring is given by the product of the number of turns and the loopcircumference. If it is not practical to make a single inductor out of asingle multi-turn coil, multi-turn segments could be joined using thepad contact points shown in FIG. 34 to produce a complete device.

[0180] One possibility with a multi-turn loop segment of FIG. 50 is toproduce more densely wound coils. For coils produced from loops ofsingle turns, layout considerations limit the loop spacing to be atleast slightly greater than the width of the spring metal in the loop.Multi-turn loops however do not have this restriction, because thesprings are made longer in the transverse coil direction to accommodatethe multiple turns. A long strip of metal can coil with a helical pitchless than its width, and the free end can overlap a contact pad eithervia the cumulative offset of the helical pitch, or by a tab the extendsfrom the free end of the spring to the pad. To prevent shorting of theoverlapping turns of the multi-turn loop, one surface, preferably, thetop surface of the spring metal can be covered or partially covered withan insulating spacer layer. This technique may require tighter controlof the radius and pitch since error in free end placement wouldaccumulate with each turn.

[0181] Other methods can be used to displace the free end of the elasticmember with respect to the takeoff point. Varying the radius ofcurvature of the coil will displace the free end in a transversedirection. The radius of curvature depends on the amount of intrinsicstress profile in the elastic member and on the mechanical properties ofthe elastic member. To obtain a desired twist, an elastic member can beformed with an intrinsic stress profile of one value in a first portionand a second value of intrinsic stress profile in the remaining portion.Another method is to put in anisotropic properties by, for example,depositing a load layer on one side of the elastic member. When theelastic member is released, the resulting coil will have two sections,each with a different radius of curvature. The effect of the twodifferent radii of curvature is that it forces the elastic member totwist.

[0182] While a coil with sections having two different radii ofcurvature can be employed to offset the landing position of the tip fromthe takeoff point of the released elastic member, a preferredconfiguration is one with three sections of different radii ofcurvature. FIG. 51 shows a plot of a roughly 0.5 mm diameter loopwherein the elastic member was designed to contact tangentially to apoint roughly 150 microns behind the take-off point of the elasticmember. The top half of the coil is composed of a larger radius than thebottom half. This has the effect of displacing the free end backwardwith respect to the take-off point. Second, by making the bottom (firstand fourth) quarters (first and third segments) of the spring have asmaller radius than the top segment, the free end of the spring contactsthe substrate tangentially. Tangential contact may be advantageous forincreasing the area of contact, and thereby lowering the contactresistance. Tangential contact may also reduce the sensitivity toplacement errors. Note that the radii for the first and third segmentsare equal, there is no need to create more than two different radii;this simplifies processing.

[0183] Another way to vary the radius of curvature is by incorporating aload layer on either the inner surface of the elastic member or theouter surface (or both). The load layer is an additional layer patternedon the elastic member to apply stress that either increases or decreasesthe bending radius. The bending radius, R, for a loaded beam can beexpressed as $\begin{matrix}{R = \frac{{Y_{o}^{2}h^{4}} + {2Y_{o}Y_{1}{{ht}\left( {{2h^{2}} + {3{ht}} + {2t^{2}}} \right)}} + {Y_{1}^{2}t^{4}}}{{\Delta \quad \delta \quad {h^{2}\left( {{hY}_{o} + {tY}_{1}} \right)}} + {6\left( {{\sigma_{1}Y_{o}} - {\sigma_{o}Y_{1}}} \right){{ht}\left( {t + h} \right)}}}} & (10)\end{matrix}$

[0184] where Y₀ is the spring modulus, Y₁ is the load layer modulus, his the spring thickness, t is the load layer thickness, Δσ is the springintrinsic stress variation, σ₀ is the net intrinsic stress in theelastic member, and σ₁ is the load layer intrinsic stress.

[0185] In the example in FIG. 51, the two radii for the first and secondsegments could be produced with the following parameters: Elastic MemberNickel alloy Member Stress Gradient  1 GPa Member Net Stress  0 GPaMember Thickness 970 nm Load Metal Gold Load Stress  0 GPa LoadThickness 180 nm

[0186] The load layer is patterned to reside only on the middle segmentof the elastic member. Note that the equation 10 assumes purely elasticbehavior, and may be approximate. Gold may relieve some of its stress byplastic flow. This may modify the thickness required somewhat. Othermaterials, with higher yield points can be substituted for gold as theload materials.

[0187]FIG. 52 shows a coil produced with a tangential offset byincorporation of a load layer. The structure of FIG. 52 may be producedin accordance with the following process. First, a release layer 301 of100 nm Ti is deposited on a substrate (not shown). Next the outer coilconductance layer 302 (which is preferably gold, but may be any othersuitable conductor) is deposited. Then the elastic member material 303,which is NiZr, is deposited on the conductor layer 302. A load layer304, which is preferably a metal layer of gold, is then deposited on theelastic member. The location of the solder pads are then masked withphotoresist, followed by plating of solder onto the solder pad areas.The solder pad mask is then stripped and the load layer is then maskedwith photoresist. This provides the location of the load layer. The loadlayer is then etched with potassium iodide and the load layer mask isstripped. Next the elastic member is masked with photoresist. Theelastic member 303 is then etched with nitric acid to form theunreleased coil. Then the coil conductance layer 302 is etched withpotassium iodide. To clear between the elastic members, the releaselayer 301 is etched, preferably by dry etching in a fluorine plasma. Theelastic member mask is stripped and then the release window is maskedwith photoresist. The release layer is removed through the releasewindow using hydrofluoric acid. If desired, the release window mask canbe stripped. When the release layer is removed, the intrinsic stressprofile in the elastic member 302 causes the elastic member to coil onitself. The load layer 304 causes a tangential offset, which enablescontact with a contact pad. Flux is applied to the solder contacts, thesolder reflows. Preferably an epoxy is applied over the resulting coiland cured. Finally the wafer is diced.

[0188] The resulting coil structure in FIG. 52 illustrates that it ispossible to create a useful coil-closing structure with as few as twosegments. FIG. 53 illustrates a top view of a completed transverselyjoined single turn loops.

[0189] The radius of curvature of the coil segment can be varied byplacing a load layer asymmetrically across a segment of the elasticmember or by introducing one or more openings asymmetrically in theelastic member prior to release. We have observed a size effect for thebending of the spring, which arises because the edges of the spring areable to relax some of the intrinsic stress. Narrower springs relax moreof the total stress at their edges than wide springs. For springs ofvaried width, or slotted springs, a theory has been worked out.Essentially, the effective biaxial modulus of the spring can be variedbetween the limits defined by Y/(1−ν) and Y/(1−ν²), where Y and ν arethe Young's modulus and Poisson ratio respectively. For typical valuesof ν the radius can be varied by about 30% by slotting the spring, orvarying its width. A similar effect is possible by placing holes(openings 162 in elastic member 160 as shown in FIG. 54a) rather thanslots (slot 172 in elastic member 170 as shown in FIG. 54b) into theelastic member; this would produce two dimensional stress relaxation.This effect can be exploited by perforating the top segment of theelastic member in order to make it bend to a larger radius. Forpractical reasons, it is better to slot the elastic member into as fewstrips as needed in the top segment in order to maximize theconductance.

[0190] Advantages of perforation are that it removes the need toseparately deposit, mask, and pattern an additional layer, such as aload layer. The process is therefore less expensive. A further advantageis that it alleviates the need to control the materials properties ofthe load layer, thus simplifying the process, and increasing yield. Theexample spring shown in FIG. 51 could have been created by slotting themiddle segment of a MoCr spring with a thickness of 1.75 microns and anintrinsic stress profile of 2.8 GPa.

[0191] A further application of perforation is to produce a controlledhelical pitch, not by growing in an intrinsic stress anisotropy asdescribed above, but by instead slotting the elastic member to produce anet torque. A slot 172 running down the length of a segment of theelastic member 170, and offset to one side, will cause the two sides ofthe segment to bend to different radii. This will pull the segment intoa helix. Other asymmetric configurations may also have utility, such asdiagonal slots or load layers, or off-center holes or load layers. Avariable radius coil would also allow higher fill factors for NiFe coresby relaxing the constraints of FIG. 38.

[0192] A significant challenge to making a useful coil is making thecoil resistance low (high Q factor). An aspect of the micro-coilsdescribed above is that high Q inductors may be created by adjusting thespring width, and outer conductor resistivity, and the outer conductorthickness. Because the skin effect confines the current to the outersurface of the coil, these factors dominate the high frequencyresistance of the inductor loop.

[0193] The resistance of the loop closure may also be limited byconnecting the free end of a loop back to a contact pad on the substratewith low resistance. Obtaining low resistance at the contact padrequires a good metallurgical junction consisting of highly conductingmaterials. Below we describe a structure and manufacturing embodimentthat achieves metallurgical junctions with low contact resistance. Coilstructures incorporating a solder pad that is reflowed to close the loophas been described above and achieves a good metallurgical junction aswell low contact resistance. Alternatively, the free end may be joinedto the contact pad by plating, either electroless or electroplating. Inthis method, the loop is formed by releasing the elastic member. Thefree end comes into either mechanical contact or proximity to a contactpad on the inductor substrate. Then, plating applies conducting materialaround both the free end and the contact pad, forming a continuous jointbetween them. In this embodiment, the application of material need notbe limited to the free and the pad areas only. Preferably, the platedmaterial has high conductivity, and is plated throughout the loop inorder to reduce the coil resistance, thereby beneficially increasing thequality factor.

[0194] The method of the invention permits process extensions. Theseprocess flows are exemplary, but other variations are possible. Forexample, certain process steps described above with respect to FIG. 52may be combined or eliminated. Layers of solder used to close the loop,could also serve as the release window for the spring release step.

[0195] The foregoing techniques can also be used to manufacture a newtype of high-Q varicaps. These varicaps use the same microspringtechnology described above, have the requisite capacitance values, andcan be integrated on chip. A varicap structure based on micro-springsallows both missing on-chip RF passive components, inductors andvaricaps, to be fabricated using the same process technology. Thesemicro-spring varicaps have the additional benefit of requiring lowerbias voltages than parallel plate MEMS capacitors. By using a spring asthe second electrode in a photolithographically patterned capacitor, andvarying the voltage between a fixed plate and the spring, thecapacitance of the structure varies.

[0196]FIG. 55 shows a cross-section of a variable capacitor employingthe micro-spring technology. A layer of metal 153 (metal 0) is firstdeposited and patterned to the desired shape on a substrate (not shown).Next a layer of a dielectric material 156 is deposited and patternedover the metal layer 153. Over the dielectric layer 156, a release layer152 is deposited. Then metal layer 151 (metal 1) is deposited over therelease layer 152. Metal layer 151 is an elastic material with aninherent stress profile built in. This inherent stress profile is builtinto the layer in the same manner as described above with respect to themicro-springs. Metal layer 151 is patterned to the desired spring shape.When the release layer 152 is patterned and partially removed, theinherent stress profile in the metal layer 151 biases the free portionof metal layer 151 away from dielectric layer 156 covering the metallayer. If an insulating material is used for the release layer 152, thedielectric layer 156 may not be necessary.

[0197] The capacitance is defined by a suspended undercut section oflength L₁ in parallel with a fixed portion of length L₀. If a DC bias isapplied between layer 153 and layer 151, electrostatic forces will causethe suspended part to bend down and increase the AC capacitance.

[0198]FIG. 56 plots the capacitance as a function of the spring lift, d,for the specific case of L₀=25 μm, L₁=100 μm, d₀=0.5 μm, capacitorwidth=500 μm, and r=500 μm. In a VCO circuit, the spring radius ofcurvature, r, could be designed so that it is identical to the loopradius of the accompanying inductor. This way both inductors andvaricaps can be fabricated in the same step.

[0199]FIG. 56 shows that the varicap capacitance changes from 2 pF to2.2 pF when the tip is deflected from 10 μm to 7 μm. This 10% tuningrange corresponds to a deflection that is well below ⅔ of the initiallift, so there is no danger of bi-stable operation where the springsuddenly snaps down. The estimated voltage required to deflect thecantilever by 3 μm is only about 10V. This low voltage is due to thecurved electrode profile, which generally requires lower drive voltagesthan more conventional actuators. For larger deflections, one canconsider tapering the spring tip to delay the onset of bi-stablebehavior. Alternatively, one can make a tapered electrode (layer 151 inFIG. 55) under a conventional spring.

[0200] Varicaps made according to the above processes exhibit excellentimmunity to vibration. The curved electrode profile allows thecantilever to be made stiffer than in parallel plate devices resultingin devices with low sensitivity to inertial forces. Under acceleration,the ratio of inertial forces to electrostatic forces is only in theorder 10⁵.

[0201] An array of variable capacitors can be arranged into a singledevice to produce a larger capacitance. FIG. 57 shows an example of alarger variable capacitor. Referring to FIG. 57 and detail FIG. 58, alarge bottom conductor layer 268 is deposited on a substrate 269.Contact 266 provides the contact for the bottom electrode, which may bemultiple electrodes connected electrically together or a single bottomconductor layer. A dielectric layer 267 is deposited on top of conductor269 followed by a release layer 270. On top of the release layer 270 isdeposited the second conductor layer 261, which is patterned into theconfiguration of parallel rows of “springs” 261, each connectedelectrically by a bus connector 263. The height of the micro-springs261, determines the capacitance, and is controlled by applying a voltagebetween the springs contact 264 and the contact for the bottomelectrodes 266. In some embodiments, if the release layer 270 is formedof an electrically insulating material, the portion of the release layerremaining beneath the first conducting layer 261 functions as thedielectric layer. This eliminates the need for depositing a separatedielectric layer. However, in most applications, it is preferred to havethe dielectric layer 267 extend completely between the first and secondconducting layers to prevent shorting.

[0202] The method of the invention can be easily applied for on-chipcircuit applications requiring an LC circuit or a tunable LC circuit.Referring to FIGS. 59 and 60, a tunable LC circuit is shown. Themicrocoil 270 connects a tunable capacitor 272 formed by plates 284 (A),and 282 (B), with common dielectric layer 286 (D). Applying a DC biasbetween plates 284 (A) and 282 (B) controls the value of thecapacitance. A DC blocking capacitor formed by plates 280 (C) and 282(B) prevents the microcoil from shorting the bias source. Note how themicrocoil 270 attaches to the DC blocking capacitor at point 290. Thecapacitor top plates 284 (A) and 280 (C) are implemented preferablyusing the same metal as for the microcoil 270. The bottom plate 282 (B)is made of an additional metal layer.

[0203] Processing is achieved economically. First the bottom conductorlayer D (286) is deposited on the substrate and etched. Then thedielectric layer 286 is deposited followed by a single release layer(not shown) which covers the area of both capacitor BC and microcoil270. A metal layer C is deposited. Then a metal layer formed of anelastic material for both capacitor layer A and microcoil 270 isdeposited and shaped. When the release layer is undercut, the microcoiland variable plate A are formed. The free ends of the microcoil areattached using one of the methods describe above.

EXAMPLE

[0204] Varicap AB with a tuning range of 500 μm by 550 μm variablecapacitor, 500 nm Si₃N₄ dielectric (ε_(r)=8)=3.5 to 22.7 pF with aminimum overlap=500 μm by 50 μm, maximum overlap at the snap-downlimit=500 μm by 320 μm. At this point the tip of plate A is down by66%.). Blocking capacitor DC of size 400 μm by 1.6 mm, 500 nm Si₃N₄dielectric layer (ε_(r)=8)=91 pF. The tuning range of both capacitors inseries=3.37 to 18.2 pF. The Micro-solenoid 270 has a 1 mm diameter, 5windings, 500 μm long=26 nH. As a result, the tuning range of the LCresonance frequency=538 to 232 MHz.

[0205] The invention provides a new type of high Q micro-inductors thatcan be integrated on Silicon ICs. Unlike most previous micro-coils, thecoil structures feature an out-of-plane architecture where the coil axisis placed parallel to the wafer surface. The out-of-plane coils addressthe problem of induced substrate eddy currents associated with in-planeinductors. It also provides a simple way to counter the increasedelectrical resistance caused by skin effects without resorting to highaspect ratio processing. The design is compatible with a large varietyof related embodiments such as coil tapping and transformers. Thisinvention supplies a major missing element in integrated RF circuitdesign.

[0206] A new type of high Q micro-spring variable capacitors andout-of-plane inductors that can be integrated on Silicon ICs has beendescribed. These varicaps when combined with inductors can beimplemented for on-chip integration of entire VCOs in superheterodynecircuits. While the invention has been described with reference tospecific embodiments, the description of the specific embodiments isillustrative only and is not to be construed as limiting the scope ofthe invention. Various other modifications and changes may occur tothose skilled in the art without departing from the spirit and scope ofthe invention.

What is claimed is:
 1. A method for forming an out-of-plane coilstructure, comprising: depositing a layer of an elastic material on asubstrate, the elastic material having an intrinsic stress profile;photolithographically patterning the layer of elastic material into anelastic member; and under-cut etching a portion of the substrate underthe elastic member to release a free portion of the elastic member fromthe substrate, an anchor portion of the elastic member remaining fixedto the substrate; wherein the intrinsic stress profile in the elasticmember biases the free portion of the coil structure away from thesubstrate, forming a loop winding and causing a free end to contact apoint on the substrate; and connecting the free end to the substrate. 2.The method of claim 1, wherein the free end contacts a point on thesubstrate other than its release point.
 3. The method of claim 1,wherein the free end contacts a point on the substrate which issubstantially the same as its release point.
 4. The method of claim 1,wherein the elastic member is formed of an electrically conductivematerial.
 5. The method of claim 1, further comprising: prior tounder-cut etching the portion of the substrate under the elastic member,forming a mechanical stop at a point on the substrate for positioningreturn of the free end.
 6. The method of claim 1, wherein the free endis connected at a point on the substrate which is offset axially fromthe anchor portion.
 7. The method of claim 1, wherein the free end isconnected at a point on the substrate which is offset opposite from theanchor portion.
 8. The method of claim 1, wherein the intrinsic stressprofile in the elastic member biases the free portion of the coilstructure away from the substrate, forming a plurality of loop windingsand causing a free end to contact a point on the substrate which isoffset axially from the anchor portion.
 9. The method of claim 1,wherein the step of depositing an elastic layer further includesdepositing at least one of a layer of an electrically conductivematerial and a layer of an elastic material having an intrinsic stressprofile; and depositing the other layer.
 10. The method of claim 1,wherein the elastic material has an intrinsic anisotropic stressgradient causing the free end to contact a point on the substrate whichis offset axially from the anchor portion.
 11. The method of claim 1,wherein the elastic material has an intrinsic shear gradient causing thefree end to contact a point on the substrate which is offset axiallyfrom the anchor portion.
 12. The method of claim 1, further comprising:prior to under-cut etching the portion of the substrate under theelastic member, depositing a load layer along a portion of a surface ofthe elastic member; wherein on release of the free portion, the loadmember causes the free end to contact at a point on the substrate whichis offset axially from the anchor point.
 13. The method of claim 1,further comprising: prior to under-cut etching the portion of thesubstrate under the elastic member, depositing a load layer along aportion of a surface of the elastic member; wherein on release of thefree portion, the load member causes the free end to contact at a pointon the substrate which is offset tangentially from the anchor point. 14.The method of claim 1, further comprising: prior to under-cut etchingthe portion of the substrate under the elastic member,photolithographically patterning at least one perforation in the elasticmember; wherein on release of the free portion, the perforation causesthe free end to contact at a point on the substrate which is offsetaxially from the anchor point.
 15. The method of claim 1, furthercomprising: prior to under-cut etching the portion of the substrateunder the elastic member, photolithographically patterning at least oneperforation in the elastic member; wherein on release of the freeportion, the perforation causes the free end to contact at a point onthe substrate which is offset tangentially from the anchor point. 16.The method of claim 15, wherein the perforation comprises a longitudinalslot in the elastic member.
 17. The method of claim 1, wherein theintrinsic stress profile along the layer of the elastic material varieswith distance from the anchor point to the free end, wherein the radiusof curvature of the coil structure varies as a function of distance fromthe anchor point.
 18. The method of claim 1, wherein the intrinsicstress profile along a first portion of the coil structure is greaterthan the intrinsic stress profile along a second portion of the coilstructure, wherein the radius of curvature of the coil structure alongthe first portion is less than the radius of curvature along the secondportion.
 19. The method of claim 1, wherein the step of under-cutetching comprises the step of: photolithographically patterning arelease window around the free end of the elastic member, and applyingan etchant formulated to etch the substrate under the elastic member,the free portion of the elastic member being released from thesubstrate.
 20. The method of claim 18, wherein the step of depositingthe layer of the elastic material comprises the step of sputterdepositing a plurality of sub-layers of a metal alloy, each of theplurality of sub-layers deposited at a different selected plasma gaspressure, each different selected plasma gas pressure creating acorresponding different level of intrinsic stress in a corresponding oneof the plurality of sub-layers.
 21. The method of claim 20, wherein abottom-most sub-layer has an intrinsic compressive stress, andsub-layers above the bottom-most sub-layer have increasing intrinsictensile stress relative to the bottom-most sub-layer.
 22. The method ofclaim 1, further comprising connecting the free end to the substrate viasoldering.
 23. The method of claim 1, further comprising connecting thefree end to the substrate via plating.